We propose to study the differential response of regressed prostatic epithelium of the castrated and hypophysectomized dog to individual sex hormones and the synergistic action of prolactin. The major objectives of this investigation will be to specify those morphologic features of the responding epithelial cells and patterns of sex hormone uptake, retention and metabolism by the affected gland that will distinguish estrogen-from androgen-controlled differentiation and direct from indirect effects. We shall apply the experience gained with the canine model to a pilot study of human prostate carcinoma in order to explore the differential sex hormone responsiveness of neoplastic cells. To accomplish these objectives we intend to (1) establish conditions for estrogen-induced proliferation of the regressed epithelium (2) locate sex radiosteroid tracers in the responding epithelium by dry-mount autoradiography after in vivo and in vitro exposure (3) study sex hormone disposition in the regressed and hormonally-altered canine prostate after intra-arterial infusion of the radiostubstrate or in culture (4) investigate cell surface alterations in hormonally-modified and neoplastic prostate epithelium.